A. Field of the Invention
The present invention relates to a lapping apparatus for polishing an object which requires its surface to have a mirror finish and a uniform thickness, such as semiconductor wafers and CD disks, by using abrasive cloths, and more particularly, to an abrasive cloth dresser which is provided in a lapping apparatus to remove slurry-like reaction products of (a) an abrasive soaked into an abrasive cloth and (b) particles scraped off from an object during the polishing with the abrasive cloth by using a high-pressure fluid.
B. Description of the Related Art
The related art will be described in connection with the case of polishing a semiconductor wafer as an object to be polished which requires a mirror finish with a uniform thickness.
Semiconductor wafers produced by cutting single crystal silicon into thin plates require their surfaces to be polished to have a fine mirror finish and a uniform thickness before forming semiconductor circuits.
To this end, a lapping apparatus has been used in which a piece of abrasive cloth, made of felt, is stuck to a rotating base plate (or a platen) and a plurality of heads including a plurality of semiconductor wafers fixed thereto are rotated, causing the abrasive cloth and the semiconductor wafers to rotate relatively in contact relation. In this condition, one side of each of the semiconductor wafers is pressed against the abrasive cloth while an alkaline solution containing abrasive grains, such as fine silica particles, is supplied to the rotating abrasive cloth. As a result, the surfaces of the semiconductor wafers are polished by the process of mechanochemical polishing so that the wafer surfaces have a mirror finish.
However, after the polishing is repeated many times, the slurry-like reaction products of the abrasive and the particles scraped off from the semiconductor wafers during the polishing are soaked into the abrasive cloth to render it loaded or fouled with the reaction products, resulting in a reduced polishing ability of the abrasive cloth.
Also, because a central area of the abrasive cloth rotates at a lower circumferential speed than an outer peripheral area, the reaction products once soaked into the abrasive cloth are hard to separate out and a larger amount of reaction products tends to soak into the central area of the abrasive cloth. This lowers the pumping action of the abrasive cloth which is caused by intermittent pressing by the semiconductor wafers and restoring forces of the abrasive cloth itself in the direction of thickness. Corresponding to such a lowering in the pumping action, the circulating effect of reaction products is reduced to relatively diminish the amount of abrasive cloth and hence lower the polishing speed. This results in the problem that the polishing surfaces of the semiconductor wafers are tapered, making it impossible to polish the semiconductor wafers into a uniform thickness.
As a method of solving the above problem, it has been attempted to dress the abrasive cloth by stopping the polishing work when loading of the abrasive cloth has reached a certain degree, and rubbing a diamond or ceramic whetstone against the abrasive cloth while a high-pressure pure water is jetted to the same, thereby removing the reaction products from the abrasive cloth.
Such a dressing work is effective to a certain extent in removing the reaction products deposited on the surface layer of the abrasive cloth, the abrasive grains of which polishing effect are reduced, such as fragments of the abrasive cloth chipped off by the dressing, particles scraped off from the whetstone, etc., but the effect is not sufficient to remove the reaction products soaked into an inner layer of the abrasive cloth. As a result, the pumping action cannot restore sufficiently, even after the dressing, to such an extent that semiconductor wafers are surely prevented from tapering in the polishing thereof. In some cases, the surfaces of semiconductor wafers might be damaged by the particles which are not successfully removed from the abrasive cloth, but are separated out to the surface layer.
In order to avoid the above problem, the abrasive cloth must be frequently replaced right before the abrasive cloth has been loaded. This solution, however, raises the problem of increasing the number of abrasive cloths used, and reducing efficiency of the polishing work due to frequent replacement of the abrasive cloths.
As a method of solving the above problem, a lapping apparatus has been proposed which includes a dresser for jetting pure water under high pressure of about 500 kg/cm.sup.2, for example, from a jet nozzle to the polishing surface of the abrasive cloth during the polishing work, so that the reaction products residing in the inner layer of the abrasive cloth are caused to come out for removal with impacts of the jetted water (see, e.g., Japanese Patent Laid-Open No. 3-10769 filed by the applicant).
In the proposed abrasive cloth dresser, an anti-scattering cover is provided to surround the whole of a plurality of nozzles for preventing the reaction products removed by the spray of the high-pressure pure water from being sprung out by the jetted high-pressure pure water and scattered to surroundings.
C. Problems to be Solved by the Invention
However, the proposed dresser has been found problematic in that the anti-scattering cover surrounding the plurality of nozzles has a large size, and that partly because of mutual relation among sprays of the high-pressure pure water jetted out of the plurality of nozzles, the pure water and the reaction products remain in the anti-scattering cover so as to coat the surface of the abrasive cloth, and the remained pure water absorbs energy of the high-pressure pure water jetted out of the nozzles, thereby reducing the ability of removing the reaction products from the abrasive cloth.
Further, the reaction products soaked into the abrasive cloth are different in amount depending on locations over the abrasive cloth. From this viewpoint, another problem has been found in that if the reaction products are removed from the entire abrasive cloth under the same conditions, dressing efficiency would not be good and the occurrence of a taper in thickness of semiconductor wafers would not yet be prevented satisfactorily. In particular, such an arrangement that the high-pressure pure water is supplied to the plurality of nozzles through the same supply pipe is not suitable for, e.g., a method of selectively removing the reaction products depending on a fouling degree of the abrasive cloth, which method is believed to be more preferably in consideration of that loading (fouling) of the abrasive cloth depends on locations.
The above-discussed problems are not limited to the polishing of semiconductor wafers, but similar problems are also encountered in polishing the CD surface, the glass surface for liquid crystals, etc.
While the above description is made in connection with the case of using high-pressure pure water to remove the reaction products, the reaction products can also be removed by using an other high-pressure fluid than pure water and, the above-discussed problems are similarly encountered in such a case.